Failure Modes of Tuff Samples from Leg 131 in the Nankai Accretionary Wedge

Abstract

Triaxial deformation tests were conducted for Leg 131 core samples (tuff) having porosities ranging from 25% to 40% under effective pressures up to 20 MPa. Samples saturated with distilled water were deformed under drained conditions at a nominal strain rate of 2.6 × 105/s. The porosity change was monitored, and the loading ram was stopped at different stages of deformation to allow for measuring permeability using the pulse decay technique. Compressional velocities were measured in nominally dry samples to infer the deformation-induced porosity change. Observation of the failure modes of the deformed samples showed that there exists a boundary, dependent upon both porosity and effective pressure, that separates the brittle and ductile regimes. Samples that are more porous and under a higher effective pressure deform more ductilely than less porous samples under lower effective pressures. The predicted behavior of the Nankai prism, based on the failure mode transition map and porosity data from Leg 131, is consistent with the observation of macroscopic homogeneous deformation by Cataclastic flow. The presence of localized brittle failure at the toe of the Nankai prism (indicated by the documented occurrence of discrete faults, shear bands, and zones of breccia and scaly fabric) can be explained by locally induced pore pressures in excess of hydrostatic. We speculate that locally high pore pressures associated with shear zones reduce the effective pressure to below the critical value by enough to move the failure mode into the brittle regime, thereby explaining the discrete character of faults in a region that is otherwise macroscopically ductile.